This invention relates to a toner for developing electrostatic latent image formed by electrophotography, electrostatic recording or electrostatic printing.
With an increasing requirement for energy saving in view of environmental protection, there is an increasing demand for toner capable of being fixed at a low temperature. Thus, the use of a polyester resin as a toner binder in place of a customarily employed styrene-acrylate copolymer is now proposed, because the polyester resin can provide toner permitting relatively low temperature fixation and having relatively good preservability. With a view toward improving the low temperature fixation, proposals have been made to use a polyester having a low glass transition point or a polyester having a low molecular weight. The former technique, however, adversely affects the preservability of the toner at an elevated temperature. The latter technique results in occurrence of hot offset, namely deposition of fused toner onto a heated roll used for fixing a toner image on a transfer medium such as paper.
Hot offset can be prevented by applying an oil to a surface of the heated roll. However, since an oil tank must be installed in the image forming apparatus, compactness of the apparatus is not attainable. Thus, in lieu of applying an oil, incorporation of a wax in toner has been proposed. During fixation, the wax melts and exudes from the toner particles to exhibit its releasing properties. Rather than being homogeneous mixed in the toner binder, therefore, the wax is desired to form a plurality of domains and to be dispersed in the matrix of the binder. As the binder, a resin incompatible with the wax should be preferably used.
In a grinding step for preparing toner particles containing a wax and an incompatible binder resin, stresses are predominantly exerted at interfaces between the wax particles and the binder resin. Therefore, the wax is apt to be present on outer surfaces of the toner particles. Further, the greater the dispersion diameter of the wax particles, the larger becomes the amount of the wax exposed to outer surfaces of the toner particles. While the presence of the wax on outer surfaces of the toner is advantageous from the standpoint of releasability, fluidity of such toner becomes unsatisfactory. Further, a filming problem, namely deposition of the wax onto carrier particles, a photoconductor or a charger is apt to occur.
Polyester resins are less compatible with a wax than styrene-acrylic copolymer resins are. When the polyester resin is of a type which does not require low temperature fixation (e.g. a high softening point polyester), kneading of the polyester resin together with a wax can be carried out with such a high shear force that the wax is finely dispersed in the polyester resin matrix. In this case, even when the wax is excessively finely divided, the polyester resin having a large cohesive force can provide releasability. When the polyester resin is of a type which requires low temperature fixation, however, the kneading cannot be performed in high shear conditions so that the wax is not finely dispersed in the resin. Therefore, the durability and transferability of the resulting toner are adversely affected by the wax.
In the case of a full color toner, the binder resin is required to have a lower softening point as compared with that for a black toner in order to obtain good gloss and good OHP transmissivity. Therefore, the wax of color toner is less easily finely dispersed in the binder resin.
To cope with the above problem, the use of a polystyrene resin to which a styrene resin has been grafted as a dispersing agent has been proposed (JP-A-H08-101526, JP-A-H05-188636, JP-A-H06-250432 and JP-A-H09-127718). While a wax can be finely dispersed in the resin matrix when such a dispersing agent is used, the amount of the wax must be increased in order to attain satisfactory releasability. As a consequence, the durability and transferability of the resulting toner are adversely affected by the wax.
JP-A-H07-084407 and JP-A-H10-207116 propose a toner produced by suspension-polymerizing a composition containing a wax, a polymerizable monomer, a colorant, etc. In such a toner, since the hydrophobic wax is surrounded by the binder resin, it takes a relatively long time for the wax to exude from the toner surfaces. Thus, high speed image formation is adversely affected.
In the case of a full color toner, the toner image must have a proper gloss. Therefore, during the fixation of a toner image, the toner is sufficiently fused to obtain smooth image surfaces. Hitherto, a heat roller is used to fix toner images on an image receiving sheet such as paper. The heat roller-type fixation has problems because a relatively long time is required to heat the roller to a predetermined heating temperature and because the heat roller must have a large heat capacity to maintain the heating temperature. On the other hand, a recently proposed belt-type fixation is free of such problems. Further, since the paper is in contact with a flat portion of the belt, occurrence of offset and curl can be reduced. Additionally, since the heating point and discharge point are spaced apart a sufficient distance to provide an increased fixing time inclusive of preheat time and nip time, the fixation temperature may be reduced without reducing image production speed. However, since the belt should be thin in order to reduce the heat capacity and since the pressure at which the belt is contacted with the paper should be low in order to prevent formation of wrinkles, the belt-type fixation is less advantageous with respect to the formation of images having high gloss.
It is, therefore, an object of the present invention to provide a toner which has permits low temperature fixation while preventing hot offset.
Another object of the present invention is to provide a toner which has good transferability and high developing efficiency.
It is a further object of the present invention to provide a full color toner capable of giving color images with satisfactory gloss and transparency.
It is yet a further object of the present invention to provide a toner of the above-mentioned type which has good heat resistance and preservability.
It is a further object of the present invention to provide a toner which can be produced with high efficiency, in particular with high pulverization efficiency.
In accomplishing the foregoing object, the present invention provides a toner for developing electrostatic latent images, which includes a matrix of a first resin selected from the group consisting of polyester resins, polyol resins and mixtures thereof, a plurality of domains of a second resin dispersed in said matrix and containing wax in an amount of 2-15% by weight based on the weight of the toner. The matrix is substantially free of domains of the second resin having a maximum diameter of greater than 2.0 xcexcm. A part of the domains containing wax having a maximum diameter of at least 0.2 xcexcm constitutes a part of an outer surface of the toner.
It has been found that, when the toner has a construction as defined above, wax having a proper particle size can be finely dispersed in the first resin matrix while maintaining the amount of the wax exposed on an outer surface of the toner small. Namely, shearing forces exerted during pulverization for forming the toner are predominantly applied to the interface between the first and second resins. Thus, the islands of the second resin are present in a relatively large amount on outer surfaces of the resulting toner particles, while the amount of the wax present on outer surfaces of the toner particles is small. However, the wax is not prevented from exuding from the toner surfaces during fixation because it is present in portions adjacent to or near the outer surfaces of the islands, namely adjacent to or near the outer surfaces of the toner particle.